From a functional point-of-view, an electrohydraulic servovalve is a device for converting an electrical signal into a hydraulic output. These servovalves are often employed in larger control systems, and it is generally desirable to have the hydraulic output be substantially proportional to the elctrical input.
Within the broad family of servovalves, there are many different types and species, each offering unique operating characteristics and features. For example, in a "flow control" valve, flow is substantially proportional to the supplied electrical current at constant load. In a "pressure control" valve, a different pressure output is substantially proportional to such supplied current. A "pressure-flow" (PQ) control valve has hybrid or intermediate properties. A "dynamic pressure feedback" (DPF) valve functions as a "pressure control" valve under dynamic conditions, but as a "flow control" valve under static conditions. Other types include "static load error washout" (SLEW) valvesl, and "acceleration switching" (AS) valves. These various types of valves are comparatively illustrated and described in Technical Bulletin 103. "Transfer Functions for Moog Servovalves", Moog Inc. (1965).
All of these different species are of the two-stage typek in that a first- or pilot-stage is used to control the operation of a second- or output-stage. The first-stage typically includes a torque motor, which positions a flapper between two opposed nozzles in response to the magnitude and polarity of an electrical current supplied to the coil(s) of the torque motor. The position of the flapper relative to the nozzles is used to create a differential pressure, which is then used to selectively shift a second-stage valve spool relative to a body. Additional details of such servovalves and torque motors employed therein, are shown and described in U.S. Pat. Nos. 2,931,389, 2,964,059, 3,023,782, 3,095,906, 3,455,330, 3,612,103, 3,542,051, and 3,464,318, the aggregate disclosures of which are hereby incorporated by reference.
There is an every-increasing need to improve the performance of such servovalves. In torque motors which have been developed heretofore, an electrical current is supplied to a coil to produce a magnetic field which acts on an armature. However, there is an inductive lag during which the coil develops the magnetic field in response to the supplied current. This lag, or time delay, either alone or in combination with other factors, reduces the dynamic response of the torque motor, and hence that of the valve and any system in which the valve is employed.
Piezoelectric devices and mechanisms are well known. These devices will change dimensionally in response to a supplied voltage. However, such piezoelectric actuators have, upon information and belief, been heretofore regarded as practicably unsuited for widespread use in torque motors and valve actuators, principally because of relatively small magnitude of such dimensional change. See, e.g., Blackburn et al, Fluid Power Control. The M.I.T. Press (4th Printing 1972) [at p. 323]. Thus, such piezoelectric actuators have been largely limited to the types and applications disclosed in U.S. Pat. Nos. 4,022,166, 3,524,474, 4,441,526, and 4,492,246.